Data compression techniques enable large quantities of data to be transmitted over relatively small bandwidths. The algorithms used in a compression system depend on the available bandwidth or storage capacity, the features required by the application, and the affordability of the hardware required for implementation of the compression algorithm (encoder as well as decoder). The moving pictures expert group-type 2 ("MPEG-2") compression standard, which is hereby incorporated by reference, is a well-known technique for compressing video data. Motion estimation, which is the process video encoders use for calculating motion vectors, is generally considered the most expensive part of the encoding process. Similarly, motion estimation is likely to be the most expensive part of a transcoding process in which a video sequence is decoded, and then re-encoded with new parameters. With the advent of high-definition television ("HDTV"), this is a significant concern since television studios will be required to transcode MPEG-2 bitstreams of picture data from one format to another. For example, television studios accommodating the HDTV standard must be able to transcode an MPEG-2 bitstream from one group of pictures ("GOP") structure to another, from one picture size to another, and/or from one bit rate to another. There may be instances where these studios will need to transcode MPEG-2 pictures from field pictures to frame pictures, or from frame pictures to field pictures. There may also be instances where the studios will need to transcode MPEG-2 pictures from an interlaced sequence to a progressive sequence, or from a progressive sequence to an interlaced sequence. Transcoding may also be used to perform a "cropping" function wherein an input represents a main image, and the transcoded output represents a sub-image within the main image.